1. Field of the Invention
The present invention relates to an occupant restraint system and a method for restraining an occupant in a seat of a vehicle.
2. Discussion of the Background
In the related art, there is a regulation which requires the provision on a vehicle such as a motor vehicle of an occupant restraint system such as a seat belt which prevents a forward movement of an occupant seated in a seat of the vehicle by virtue of inertial force when the vehicle is involved in a head-on collision. In general, while the safety of the occupant at the time of head-on collision is increased due to the occupant restraint system applying tension to the seat belt at all times, since the occupant is restrained in the seat at all times by the seat belt, the degree of comfortableness felt by the occupant gets deteriorated. To cope with this, there has been known an occupant restraint system including a head-on collision prediction unit for predicting a head-on collision of the vehicle and detecting a head-on collision of the vehicle (for example, refer to Japanese Patent Unexamined Publication No. Hei. 6-286581). The contents of this publication are incorporated herein by reference in their entirety.
As shown in FIG. 7, such an occupant restraint system 100 includes a seat belt 91 which can be worn by an occupant P seated in a seat 26 of a vehicle, a reversible pre-tensioner unit PT1 and a non-reversible pre-tensioner unit PT2 which are both adapted to apply a tension to the seat belt 91, an inter-vehicle distance sensor 120 and an acceleration sensor 110 which are both adapted to detect states of the vehicle, and a control unit 170 for predicting and detecting a head-on collision of the vehicle based on signals outputted from those sensors 120, 110 and activating the reversible pre-tensioner unit based on the prediction of a head-on collision-while activating the non-reversible pre-tensioner unit based on the detection of a head-on collision.
As shown in FIG. 8, prediction and detection of a head-on collision of a vehicle M2 on to a vehicle M1 are carried out based on an inter-vehicle distance Lc detected by the inter-vehicle distance sensor 120 and the acceleration of the vehicle M2. In the occupant restraint system 100 configured as such, when a head-on collision is predicted and detected, the occupant is restrained in the seat for preparation for a head-on collision, and the restraint of the occupant in the seat is not carried out on other occasions. Due to this, since the tension of the seat belt has already been increased so as to restrain the occupant in the seat when the vehicle is about to be involved in a head-on collision, the safety of the occupant is increased, and when there is no risk of the vehicle being involved in a head-on collision, the tension on the seat is mitigated so as to reduce the restraining force with which the occupant is restrained in the seat to thereby enhance the degree of comfortableness felt by the occupant.
However, the related-art occupant restraint system 100 includes the sensor which predicts and detects a head-on collision of the vehicle but does not include a sensor which direct predicts and detects a rollover of the vehicle. Due to this, the occupant restraint system 100 activates the non-reversible pre-tensioner unit in order to restrain the occupant in the seat in an early state of a rollover. Even in the event that the occupant restraint system 100 activates the non-reversible pre-tensioner unit, however, there may result a case where the vehicle does not roll over eventually. This causes a problem that the occupant has to be restrained in the seat to thereby be made to feel uncomfortable every time the false rollover warning is raised.